Recently, expectations for optical resins are getting higher, where there is a need for resins with excellent heat resistance and light resistance, low absorptivity and high transparency. However, conventional optical resins do not have the above required properties in balance at a high level, and thus have various disadvantages as an optical resin.
For example, for optical resins with high transparency, conventionally polymethylmethacrylates, polycarbonates and the like have been used. Polymethylmethacrylates have excellent optical properties such as high transparency and low birefringence, but are problematic in that the dimensions are apt to change due to their high absorptivity and also they have low heat resistance. On the other hand, polycarbonates have a high glass transition temperature (Tg) and excellent heat resistance, but are problematic in that their absorptivity is somewhat high and they are prone to hydrolyzation by alkali.
For optical resins with high heat resistance, low absorptivity and excellent transparency, hydrogenated ring-opening polymers of norbornene-based monomers, or addition copolymers of norbornene-based monomers with ethylene have been known (Patent documents 1-4). However, it is not always easy to prepare polycyclic monomers of tetracyclododecenes used as the norbornene-based monomers, and it is necessary to use chlorides of rare metals such as molybdenum and tungsten for the polymerization catalysts.
As an optical resin that overcomes the above problems, β-pinene-based polymers have been proposed (Patent document 5, Non-patent documents 1-2). β-pinene-based polymers are materials having high heat resistance and low absorptivity. Further, they have been in the spotlight as carbon neutral materials that inhibit the emission of carbon dioxide, which has been a recent problem. However, no β-pinene-based polymers have been reported that have both higher heat resistance and light resistance. That is, the copolymers of β-pinene with indene, which are described in Patent document 5 (Examples 7 to 12), have high heat resistance, but are problematic in that they are prone to oxidization and degradation due to the residual β-pinene-derived olefinic double bonds or indene-derived aromatic double bonds, and are thus easily colored by light or heat. In addition, although there is an example described in Non-patent document 1 of a β-pinene-based polymer having high light resistance, hydrogenation is insufficient, and during the addition of contacted hydrogen by conventional hydrogenation catalysts, the β-pinene-derived 6-membered rings are aromatized to some degree when hydrogen is added, resulting in insufficient heat resistance.